a) Field of the Invention
This invention relates to a rolling bearing for a high-speed rotating equipment employed in a field where high speed and high safety are required, such as a medical, food-processing or aerospace equipment.
More specifically, the present invention is concerned with a rolling bearing as a principal element of a high-speed rotating equipment such as a dental high-speed rotating cutter (dental air turbine hand piece), which assures stable high-speed rotation, has high biosafety, and is equipped with durability sufficient to withstand sterilization treatment (autoclaving) under high temperature and high pressure.
b) Description of the Related Art
A high-speed rotating equipment, especially a high-speed cutter for rotating a cutting tool at high speeds is generally constructed of a rotary shaft for fixedly holding various cutting tools thereon, a drive unit for rotating the rotary shaft, and a bearing unit rotatably supporting the rotary shaft.
As a high-speed cutter of this type, an odontotherapeutic high-speed cutter (i.e., air turbine hand piece) can be mentioned by way of example.
Bearing units for the above-mentioned odontotherapeutic high-speed cutters (i.e., air turbine hand pieces) are known to include those having a ball bearing system making use of balls (rolling elements) and those equipped with a (non-contact) air bearing system making use of an air bearing.
Paying attention, for example, to the bearing mechanisms of dental air turbine hand pieces, two types of air turbine hand pieces are known, one being of the ball bearing turbine type and the other of the air bearing turbine type.
The former type, i.e., the ball bearing turbine type can be considered to be a high-speed rotation type for approximately 200,000 to 400,000 rpm, while the latter type, that is, the air bearing turbine type can be considered to be a super high-speed rotation type for approximately 300,000 to 500,000 rpm.
It is however to be noted that the above-described revolution speeds of the ball bearing turbine type and the air bearing turbine type are general values. For example, the dental air turbine hand piece already proposed by the present inventors (U.S. Pat. No. 5,562,446) is of the ball bearing turbine type. Nonetheless, it has high performance so that super high-speed rotation can be achieved.
To facilitate the understanding of the conventional art and this invention, a description will now be made about the construction of an equipment to which a bearing according to the present invention, said bearing comprising a lubricating oil and a porous retainer, is applied, namely, of a dental high-speed cutter (i.e., a dental air turbine hand piece).
FIG. 1 to FIG. 2 show the construction of the dental air turbine hand piece, in which FIG. 1 is a perspective view illustrating the overall construction and FIG. 2 is a cross-sectional view illustrating the internal construction of a head portion and a neck portion in particular.
As is depicted in FIG. 1, the dental air turbine hand piece designated generally by letter A is composed of a head portion H, which carries a cutting tool B(5) fixedly held on a rotor shaft (drive shaft) of an air turbine, and a grip portion G.
A neck portion N of the grip portion G is connected to the head H, and is internally provided with means for supplying compressed air to the air turbine arranged within the head portion H and also for discharging compressed air from the air turbine.
FIG. 2 illustrates the internal construction of the head portion H and the neck portion N of the dental air turbine hand piece A.
As is illustrated in the figure, in the head portion H, a turbine rotor shaft 3 with turbine blades 2 disposed at a peripheral edge portion thereof is arranged within a chamber 11 of a head 1, and the turbine rotor shaft 3 is rotatably supported within the head 1 by way of a bearing unit 4.
The head 1 is composed of a head main part 12 and a cap part 13. Within the head main part 12, the bearing unit 4 is arranged to rotatably support the turbine rotor shaft 3. To perform treatment, the cutting tool 5 is fixedly held in a bore which is formed through the turbine rotor shaft 3 along a central axis thereof. Incidentally, the cutting tool 5 is provided on a peripheral side wall thereof with a chuck 51 for holding the cutting tool 5 in place within the bore.
The bearing unit 4 is of the ball bearing type and is constructed of an inner ring 41, an outer ring 42, rolling elements 43 and a retainer 44. The bearing unit 4 may be provided on an outer periphery or side wall thereof with O-rings for providing the bearing unit with self-centering function and/or with known wave washers for enhancing the rigidity of the shaft.
A main part 6 of the neck portion N is provided with an air supply passage 7 and an air inlet 71 for supplying compressed air to the turbine blades 2 arranged within the chamber 11 and also with air discharge passages 8,9 and air outlets 81,91 for discharging compressed air from the chamber 11.
In the above-described internal construction of the dental air turbine hand piece A as illustrated in FIG. 2, the means for supplying and discharging compressed air is the one already proposed by the present inventors (U.S. Pat. No. 5,562,446) and is of a new construction totally unseen in the conventional art.
Accordingly, FIG. 2 contains other reference symbols in addition to those referred to in the above to describe the individual elements (members). Although a description of these additional symbols is omitted herein, the construction of the conventional dental air turbine hand piece can be easily understood on the basis of FIG. 2.
The dental air turbine hand piece A--which is equipped with the air supply and discharge means shown in FIG. 2 and already proposed by the present inventors--belongs to the category of conventional hand pieces with a rolling bearing built therein, but makes it possible to obtain rotation of an extremely high speed and hence a large torque.
In the above-described dental air turbine hand piece of the ball bearing type, its bearing unit is in the form of a miniature bearing unit. Since the turbine rotor shaft rotates at a high speed of approximately 200,000 to 400,000 revolutions per minute, the temperature inside the bearing unit becomes high and further, the bearing unit is exposed to a large stress. For a lubricating oil to be applied to a bearing which is used under the above-described severe conditions, it is therefore extremely important to control its quality and properties.
Further, the above-described dental air turbine hand piece of the ball bearing type is used in the oral cavity. Accordingly, the dental air turbine hand piece is used by spraying or dropping a lubricating oil to the bearing unit, in other words, is used under an environment of minimum lubrication, and is subjected to high-pressure and high-temperature treatment for sterilization and disinfection (which is also called "autoclaving", which is applied under the following conditions: steam pressure, 2.4 kg/cm.sup.2 ; temperature, 135.degree. C.; time, 5 minutes).
As a rolling bearing for use in the above-described dental air turbine hand piece, one capable of meeting conditions similar to those mentioned above is therefore required. In particular, as a retainer which acts as an important element in a lubrication system, one capable of satisfying conditions similar to those mentioned above is required.
Further, the lubricating oil for use in the above bearing is also required to have properties sufficient to withstand such severe use conditions, such as oxidation resistance.
Conventional examples of the above-described retainer for rolling elements (balls) include those made of polyimide resins or fiber-layer-containing phenol resins from the viewpoint of the above-described property requirements. These conventional retainers are however not considered to be sufficient.
As lubricating oils applied to rolling bearings in high-speed rotating equipments, on the other hand, a variety of lubricating oils have been used or proposed.
For example, it is widely practiced to supply a lubricating oil by spraying it with flon or LPG. As lubricating oils, those making use of refined mineral oils, such as paraffin, as base oils are known.
The above-described lubricating oils are typically of the petroleum base and are prepared by fractionating and refining petroleum into various fractions and incorporating additives, such as an oxidation inhibitor, therein as needed.
As base oil components of the above-described lubricating oils, there are known synthetic oils such as glycol esters and low molecular weight polyolefins in addition to natural mineral oils. Furthermore, edible oils such as animal oils and vegetable oils are also known to be in use as lubrication oils for precision machines, machine tools, ship engines and the like. In general, however, these edible oils are used by being added in proportions of from 10 to 20 wt. % to lubricating oil of the mineral oil base.
Incidentally, these edible oils have a problem in oxidation resistance. It is therefore the common practice to use such edible oils in combination with one or more of various oxidation inhibitors.
In view of the above-described severe use conditions for dental air turbine hand pieces, dental air turbine hand pieces equipped with retainers impregnated with fluorinated oils, which have excellent heat resistance, permit sterilization and disinfection (autoclaving) and have superb lubricity, have been proposed recently in Japanese Patent Publication (Kokoku) No. HEI 5-43884 and Japanese Utility Model Application Laid-Open (Kokai) No. HEI 7-10553.
Incidentally, the retainers are in the form of porous members obtained by sintering powder of a polyimide resin.
The fluorinated oils have properties such that they are inactive, are excellent in heat resistance, chemical resistance and solvent resistance and, even when exposed to high temperatures, do not form a solid deterioration material. It is therefore possible to consider that the above-described proposed dental air turbine hand pieces have used these properties for the retainers.
Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 discloses an embodiment of impregnating with a lubricating oil a snap retainer of a ball bearing in a dental air turbine hand piece to be described briefly hereinafter, although its direct subject matter is the snap retainer itself.
Specifically, in the dental air turbine hand piece disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 referred to in the above, the retainer of the ball bearing is characterized in that:
(i) the retainer is a snap retainer, in which a ball-holding pocket is formed on one side of a synthetic resin cylinder having a fabric-made fiber layer therein, and chamfered portions are formed on opening-side end edges of the pocket; and PA1 (ii) the fiber layer of the retainer is impregnated with a lubricating oil. PA1 (i) biosafety (reduced harm to the body), PA1 (ii) environmental conservation (safety), PA1 (iii) heat resistance (the possibility of sterilization and disinfection by autoclaving), and PA1 (iv) durable lubrication system. PA1 (i) is composed of a molded porous resin body having a porous portion of a communicated structure (communicated porous portion) in least a part thereof, and PA1 (ii) has been obtained by impregnating the porous portion of the communicated structure with a lubricating oil comprising a nondrying vegetable oil.
Incidentally, the snap retainer of the ball bearing disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 is to improve the balance of rotation by the feature (i), thereby making it possible to protect the retainer from wearing and also to avoid an increase in rotating torque, both of which would otherwise occur due to contact between the retainer and the outer ring.
However, the specific composition of the lubricating oil impregnated in the retainer is not clear in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790. As a matter of fact, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 does not disclose whatsoever the specific composition of the edible oil, although it describes an embodiment of charging an edible oil into a housing in the description of the conventional art. It is the present inventors' belief that the lubricating oil or edible oil disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 is still no better than the level of the conventionally-proposed lubrication oils in view of the technical level in the present field of art.
Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-212179 discloses mixing of ceramic powder in a lubricating oil to reduce the replenishing frequency of a lubricating oil to a bearing of a dental air turbine. This is a proposal based on a finding that advance mixing of ceramic powder in a lubricating oil can improve the life time of the lubricating oil.
The above-described various lubricating oils, which have been proposed to date, are still accompanied by various drawbacks which should be improved in order to permit their application in high-speed rotating equipments such as odontotherapeutic air turbine hand pieces having rolling bearings.
For example, the above-described lubricating oils of the mineral oil base, such as of the liquid paraffin base, or of the synthetic oil base or the lubricating oils obtained by adding edible oils to them still involve matters to be improved from the viewpoint of biosafety and environmental conservation.
In addition, the fluorinated oils proposed in the above-described Japanese Patent Publication (Kokoku) No. HEI 5-43884 and Japanese Utility Model Application Laid-Open (Kokai) No. HEI 7-10553, such as perfluoropolyether (PFPE) and perfluoropolyalkyl ether (PFAE), are excellent in heat resistance, chemical resistance and solvent resistance and, even when exposed to high temperatures, form no solid deterioration material. They are accordingly preferred as lubricating oils for high-speed rotating equipments. They however still involve matters to be improved from the viewpoint of environmental destruction and biosafety.
Furthermore, the above-described Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 discloses impregnation of a retainer of a rolling bearing, which is formed of a shaped member made of a phenol resin and having a fiber layer, with a lubricating oil, and suggests use of an edible oil as the above-described lubricating oil. As will be descried in detail subsequently herein, a great majority of general edible oils are drying oils and, when they become dry, they are formed into resinous solids. They are accordingly not suited as durable lubricating oils for bearings.
As has been described above, most of general edible oils are dry oils and are susceptible to oxidation. It is therefore the common practice to use them by adding a synthetic oxidation inhibitor therein. In an edible-oil-base lubricating oil composed primarily of the above-described drying oil and added with an oxidation inhibitor, it is necessary to keep in mind the potential risk that a reaction product to be formed between the oxidation inhibitor and a metal dissolved out from a bearing system may be a material harmful for the body.
According to the above-described Japanese Patent Application Laid-Open (Kokai) No. HEI 6-212179, ceramic powder is mixed in a lubricating oil to improve the life time of the lubricating oil so that the replenishing frequency of the lubricating oil to a lubrication system can be reduced. In an odontotherapeutic cutter of the high-speed rotation type (air turbine hand piece), however, the race (retainer) and balls of the bearing are ground by the ceramic powder so that a metal harmful for the body may be dissolved out, or a serious damage may be given to the bearing mechanism when the lubricating oil becomes scarce under the environment of minimum lubrication.
As has been described above, the conventional lubricating oils for rolling bearings, said oils being for use in high-speed rotating equipments, for example, odontotherapeutic high-sped cutters (air turbine hand pieces), are still accompanied by drawbacks when evaluated in view of the following properties: